Scientists are bracing for a blitz of discoveries in the last six months of Cassini’s mission at Saturn, when the plutonium-powered robot will repeatedly ram through an unexplored gap in the planet’s famous rings, then make a destructive plunge into the atmosphere in September.
The dramatic last act has been in the works since 2010, when NASA formally approved the plan, using flybys of Saturn’s moon Titan and periodic thruster burns to reshape Cassini’s orbit around the ringed planet.
Cassini’s mission will end with a Sept. 15 dive into Saturn’s hydrogen-helium atmosphere after a series of 22 close-in week-long orbits passing between the planet’s innermost icy ring and its cloud tops. The robotic spacecraft will set up for the mission’s last phase — dubbed the “grand finale” — with a flyby of Saturn’s moon Titan on April 22, followed by the first dip through the ring gap around four days later.
“In many ways, the grand finale for Cassini is like a brand new mission,” said Linda Spilker, Cassini’s project scientist at the Jet Propulsion Laboratory in Pasadena, California. “We’re going to probe Saturn’s interior, measure the magnetic field, look for the magnetic dynamo, and try and figure out why is there is so little, or perhaps no, tilt between the magnetic field axis and the spin axis of Saturn. What’s going on there?”
The docket of scientific investigations planned from April through September runs deep, but Cassini first has to survive the journey inside the rings.
“Just the feat of navigating and engineering our way through the gap between the rings and the planet, that in and of itself I consider an engineering triumph,” said Earl Maize, Cassini’s project manager at JPL.
It’s a story played out in meeting rooms, memos and on presentation slides among scientists and engineers working on many space missions. Scientists hungry for new revelations push for more data, while engineers warn of risks and dangers that could overtax a spacecraft or instrument.
Cassini’s daring last act has members of the mission’s team struggling with balancing the same dilemma: More science data or less risk?
But the calculation has changed with Cassini, which is in the last six months of a 13-year odyssey around Saturn. While managers say they want to avoid doing anything foolish with the spacecraft, the mission’s shortened time horizon has officials willing to take more risks.
The spacecraft will make its first passage through the 1,500-mile-wide (2,400-kilometre) gap between Saturn’s rings and atmosphere with its dish-shaped high-gain antenna pointing forward, blocking the orbiter’s sensitive electronics, computer and scientific sensors from collisions with ice and dust that may populate the region.
No spacecraft has ever passed through the gap, and although images do not show any signs of dust or ice in Cassini’s path, officials cannot be sure of the threat. Cassini will be moving so fast that a smash-up with a tiny grain could cause catastrophic damage.
“The innermost ring is called the D ring, and it sort of just slowly fades away into areas we can’t see,” Maize said. “We’re going into the area where we cannot see. We have really good models of the rings, and we believe we’re going to be safe, but nevertheless, there are going to be five instances where we’re going to hide behind the high-gain antenna as we go through the rings just because we’re kind of close.”
The high-gain antenna will be in its so-called “ram” position, pointed in Cassini’s direction of travel, on the first trip through the ring gap, giving ground controllers a chance to assess how much ice and dust is actually there.
Cassini will fly through the gap at slightly different locations on each orbit. On four passages from May through July, the spacecraft will be closer to the D ring, and engineers will pivot Cassini to again put its antenna in the ram position on those orbits.
“There is a possibility, and it’s higher than we normally accept, of dust collision,” Maize said in an interview with Spaceflight Now. “We’re going to try to be careful, but at the same time there is that possibility.
“If we get surprised, and we have way more dust than we thought, then we will probably hide behind the high-gain antenna much more frequently,” Maize said.
But officials are wary of big changes.
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Cassini’s flight plan is uploaded to the spacecraft in 10-week chunks, and scientific observations for this summer’s grand finale campaign are already planned in detail.
“The science has all been carefully integrated and coordinated between all the instruments, and if we start to move when we’re hiding behind the high-gain (antenna) and when we’re not, then that can be quite disruptive,” Maize said.
Another hazard awaits Cassini the other edge of the gap, where the top layers of Saturn’s atmosphere will tug on the orbiter. The slight aerodynamic forces could be too strong for Cassini’s reaction wheels, a set of spinning masses designed to keep the craft pointed with momentum.
For the mission’s final five trips through the ring gap, ground controllers will activate the probe’s rocket thrusters, burning hydrazine to keep the aerodynamic forces from putting Cassini in a tumble.
The Cassini project, first conceived in the 1980s, has cost nearly $4 billion from start to finish. Cassini launched in October 1997 from Cape Canaveral aboard a Titan 4 rocket, flew by Venus and Jupiter, and reached Saturn in July 2004, becoming the first space probe to slip into orbit there.
The orbiter dropped a European probe named Huygens to land on the surface of Titan, Saturn’s largest moon, in January 2005. Since then, Cassini has circled Saturn more than 260 times, collecting detailed imagery of Saturn’s atmosphere and mysterious hexagonal polar vortex, explored its rings in minute detail, and observed 49 of Saturn’s 62 known moons with close and long-range flybys.
Cassini was originally scheduled to collect data for four years after arriving in orbit around Saturn, but NASA extended the mission as the probe discovered that the planet and its moons demanded further study.
Titan harbors several Earth-like features, like a thick atmosphere, rivers, lakes and rain, but the liquid on Titan’s surface is not water. It’s a mix of ethane, methane and other hydrocarbons.
Saturn’s 313-mile-diameter (504-kilometre) moon Enceladus has a global ocean of water buried under ice — a finding made by scientists using Cassini. Eruptions at Enceladus’s south pole spray gas, dust, and organic material into space, and Cassini has sampled the jets in a series of flybys.
The build-up of knowledge has been incremental, with each of Cassini’s hundreds of encounters with Saturn’s moons adding another piece of the puzzle. Meanwhile, other NASA missions like the Curiosity rover and New Horizons made headlines when they landed on Mars and unveiled the face of Pluto for the first time.
“We always think we ought to be on the front page every day,” Maize said of Cassini’s legacy. “I think that it has gotten its due in the scientific community. It’s a disocvery machine.”
He cited NASA’s decision last year to ask for proposals for new missions to Saturn focusing on Titan and Enceladus. The space agency currently has no confirmed mission to Saturn after Cassini.
“The fact that they’ve actually created an Ocean Worlds program, and are allowing new missions to be proposed to Titan and Enceladus, that’s on us,” Maize said. “Those are Cassini discoveries that opened up this whole new set of horizons, that not only are there a few ocean worlds, but there may be many, and they don’t have to be big. Look at Enceladus!”
The moon Dione may also have an underground ocean, and the rest of Saturn’s motley crew of moons have their own stories.
There’s Hyperion, which rotates unpredictably, is less dense than water, and looks like a sponge or a wasp’s nest. Mimas, the closest of the major moons to Saturn, likely consists almost entirely of water ice, and its surface is scarred with a giant crater, earning it the moniker of the “Death Star.”
Two small saucer-shaped moons, Pan and Atlas, have ridges along their equators. Scientists believe the objects, each about the width of a large city, accumulate dust and ice grains as they orbit Saturn near the planet’s rings.
Cassini is currently getting some of its best views of Saturn’s smaller moons.
The spacecraft swung into an orbit in November that grazes the outer edge of Saturn’s rings, setting up for the Titan encounter in April, when Cassini will cross inside the rings. The “ring-grazing” orbit has yielded detailed views of the ring structure, as well as Saturn’s numerous moons that carve out lanes between the individual rings.
NASA released images Thursday revealing the distinct shape of Pan, drawing comparisons to ravioli or a walnut. In January, Cassini captured dazzling views of the 5-mile-wide (8-kilometre-wide) Daphnis, which plows through a 26-mile (42-kilometre) gap between Saturn’s rings, its weak gravity making waves in the neighbouring ring layers.
In the next month, Cassini will closely observe several intriguing features inside Saturn’s rings nicknamed propellers. Scientists believe the disturbances, named for famous aviators, are created by tiny unseen moonlets as small as 300 feet, or 100 metres, embedded in the rings. The spacecraft will collect some of the mission’s best images of the propellers in the coming weeks.
Saturn’s polar aurora, the dust environment around the rings, and long-range imaging of the moons Tethys and Enceladus are also on tap. Cassini will get its closest view ever of Atlas, the saucer-shaped twin to Pan, and take a picture from inside Saturn’s shadow with the planet and rings backlit by the sun, allowing scientists to produce a mosaic of the rings’ fainter components.
Then comes the mission’s last encounter with Titan on April 22. The moon’s gravity will slingshot Cassini closer to Saturn than any spacecraft in history, into an egg-shaped orbit with a high point outside the rings and a low point threading between the rings and Saturn’s cloud tops.
Researchers are eager for Saturn’s close-up, even if the mission’s end will be a “poignant moment,” Maize said.
He said most members of the Cassini team “think that they’ve landed on one of the best missions that NASA has ever flown.”
“It’s a passing and the end of an era — a great era — it’s been a great ride, and I think the the team is all deservedly very proud of their accomplishments,” Maize said. “It’s like with any good thing that has to come to an end, you don’t want it to, but we understand why.”
Cassini has tripled the duration of its planned stay at Saturn, and is now running low on fuel.
“It’s over 19 years since launch, and we’ve been at Saturn over 12,” Maize said. “The spacecraft is showing its age, in some cases.”
One instrument, the Cassini Plasma Spectrometer, stopped working in 2012, and the spacecraft is running on a backup set of rocket thrusters.
“Our reaction wheels, which we use to fine tune our attitude control, are cranky but still functioning. It’s kind of like my knees in the morning,” Maize joked.
But most of the spacecraft’s systems are still healthy.
“Given its age and the amount of stress we’ve put it through, it’s performing remarkably well,” Maize said.
So why send Cassini on a suicide mission?
Officials worry that if Cassini died before falling into Saturn, the spacecraft could plow into Titan or Enceladus, polluting the moons with toxic rocket fuel, metal alloys and potentially microbes carried from Earth.
“In a certain sense, Cassini has been a victim of its own success,” Maize said. “We found these prebiotic worlds, which almost mandate that we can’t contaminate them, so we’ve got to do something sensible with the spacecraft.”
A wreck with Cassini could throw any future discovery of life on those moons into doubt.
“The inside of Cassini is room temperature,” Maize said. “We’ve got electronics in there that are running right around 75 degrees Fahrenheit. For a hardy microbe, that’s just as comfortable as can be, so you really don’t want to leave that around Saturn.”
Navigators plotted this summer’s novel trajectory inside the rings nearly a decade ago, and NASA settled on the audacious plan after considering colliding Cassini with one of Saturn’s smaller, less habitable moons or dispatching the craft to fly by Uranus, Neptune, Jupiter, or an enigmatic Centaur object, a cross between an asteroid and a comet.
“The cruise to Uranus was something like 30 years for a fast flyby,” Spilker recalled.
A joint study by officials at JPL and engineers at Purdue University in 2009 identified a way to send Cassini through the rings with a push from Titan’s gravity.
“It was really a no-brainer at that point,” Spilker said. “The chance to go into that gap, not only for ring scientists but for the Saturn scientists, was just too much to pass up.”
“Saturn still remains so compelling that we chose to use our last ounce of fuel in the spacecraft to explore that system,” Maize said.
NASA considered steering the Pioneer 11 flyby probe through a gap between two parts of Saturn’s rings in 1979. The agency again thought about guiding the Voyager probes through the so-called Cassini Division in the rings in the 1980s, but managers opted for a farther flyby out of safety concerns.
Cassini will go even closer to Saturn than proposed on the Pioneer 11 and Voyager missions.
Maize said there is a small chance Cassini could run out of rocket fuel before Sept. 15, but its reaction wheels could keep the craft pointed to complete the bulk of this summer’s planned science campaign.
But once Cassini jumps inside the rings next month, its trajectory will naturally fall into Saturn in September, even if the spacecraft fails, runs out of fuel, or crashes into an unexpected icy debris cloud.
“There’s little chance of us actually running out of gas and sputtering to a halt,” Maize said. “It’s just how we’re going to get there.”
Spilker said scientists will measure Saturn’s gravity field better than ever before by analyzing radio signals passed between Cassini and Earth to see how much they are distorted by the planet’s gravity.
“We hope to measure the size of the rocky core in Saturn,” Spilker said Feb. 22 in a presentation to NASA’s Outer Planets Assessment Group. “And it’s this rocky core that attracted material that eventually formed Saturn. We’ll look at the interior also to try to measure the internal rotation rate.”
Cassini will be close enough to Saturn to map its gravity field with the precision to determine how deep winds penetrate inside the planet’s atmosphere.
“They could be anywhere from 300 to 3,000 kilometres (186 to 1,860 miles) in depth, and irregularities in the gravity field will provide the depth for those winds,” Spilker said.
Cassini’s grand finale orbits are similar to the elliptical laps made by NASA’s Juno spacecraft now exploring Jupiter. Spilker said information on Saturn’s interior structure learned in the coming months will be compared to data on Jupiter obtained by Juno.
Once the orbiter jumps inside the rings, scientists will be able to separate the total mass of the material inside the rings and of Saturn itself. Spilker said the uncertainty in the rings’ mass will be reduced to around 5 percent, yielding crucial clues about their origins.
“That will tell us if the rings are less massive,” Spilker said. “There are some indications that might be true, (in which case) they’re young rings, formed from perhaps the breakup of a moon or a comet that came too close to Saturn.
“If they’re more massive, then there is a possibility that they could have formed at the same time as Saturn — it’s not a given — but they could have been massive enough to survive the micrometeoroid bombardment to still be there until this day,” Spilker said.
Cassini will also sample the plasma hiding between Saturn and its rings, probing the planet’s weak radiation field.
If there are any microscopic ring particles in Cassini’s flight path, the spacecraft’s Cosmic Dust Analyzer will scoop up ice grains and directly measure their composition.
“We know the rings are 99 percent water ice,” Spilker said. “But what’s the other 1 percent or so non-icy constituent? Iron? Silicates? Organics? Tholins? A mix? We’ll get a chance to measure that directly.”
In the mission’s last five passes in August and September, Cassini will be low enough to skim the atmosphere, telling the ground team about the molecules that make up the outer rarefied layers of Saturn itself.
“On the very final orbit, we’re deep enough that we’ll actually be holding the high-gain antenna pointed toward the Earth for as long as we can,” Spilker said.
Cassini’s mass spectrometer will be gathering “in situ” data on the conditions inside the atmosphere and piping the readings back to Earth in real-time — but with a nearly 90-minute lag due to Saturn’s distance — rather than storing the measurements on recorders for playback later.
“Cassini will be delivering science data down to its last seconds of life,” Maize said.
The orbiter’s antenna can downlink information at about 140 kilobits per second. At that speed, it takes 10-to-20 seconds to transmit an image, Maize said, limiting the possibility for a final picture during the plunge.
“The pointing isn’t quite right for images anyway, although we’re still toying with the idea of maybe one more,” Maize said. “Why not? If we can rake the camera across the rings while we’re going in, it will be spectacular.”
The spacecraft’s control thrusters will be feverishly firing to keep the probe stable as long as possible as thicker streams of air tug on Cassini.
Cassini will fall into Saturn at a speed of around 78,000 mph, or 35 kilometres per second.
“As we’re sampling Saturn’s atmosphere, as long as Cassini can continue to point at the Earth, we will be sending back science data,” Maize said. “What happens is that the atmosphere will eventually push it to the point where it can’t maintain its pointing with the antenna, and it’ll probably be crushed a few tens of seconds later.”
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